Dr. Michele Rosenbaum outlines diagnostic and clinical management options for two of the most common infectious diseases seen in feline patients.
This is an exciting time in feline dermatology. Many new diseases have been discovered, and new medications are available for treatment. The days of giving a Depo-Medrol injection to treat any itchy cat, and labeling those that fail to respond as "psychogenic" hopefully are over! In this article, I will discuss feline ear mites and dermatophytosis, two of the most common infectious diseases seen in feline practice. Practical diagnostic and the latest treatments will be given for each disease, to assist you in your work-up and clinical management of these cases.
Ear mites are the most common cause of otitis in cats, especially in kittens. Otodectes cynotis, the ear mite of dogs, cats, and ferrets, causes otitis with a classic "coffee ground-like" appearance of brown-black granular discharge in the ear canals. O. cynotis is a white, fast-moving psoroptid mite (Photo 3) with four pairs of legs and a three-week life cycle. Adults have a lifespan of two months.
Photo. 3: Otodectes cynotis, the ear mite of dogs and cats.
Clinical signs are caused by irritation of the ear canal epithelium due to mite feeding behavior as they ingest epidermal debris and tissue fluid. Most cats show moderate to severe otic discharge and pruritus but this can be variable. Some cats with a large amount of discharge will have little otic pruritus, while those with minor discharge can be very pruritic. Mites are commonly found elsewhere on the body, including the neck, rump and tail. These ectopic mites may cause pruritus and dermatitis, which may resemble fleabite hypersensitivity, food hypersensitivity, or atopy. Therefore, ear mite infestation is a differential diagnosis for any pruritic cat. Ear mites are highly contagious to dogs, cats and ferrets, so all contact animals must be treated. These mites are zoonotic and can cause a temporary papular dermatitis in people.
Diagnosis of ear mites is straight forward, and involves taking a cotton swab of ear canal debris and rolling it onto a glass slide with a drop of mineral oil added to spread material evenly. The mites are large and often readily apparent. Often mites can be seen on otoscopic examination as moving white specks among the otic discharge.
Treatment begins by cleaning the ear canals with mineral oil or a cerulytic agent. There are numerous otic medications available for the treatment of ear mites. All contact animals must be treated. Otic parasiticides such as TresadermÂ® (neomycin sulfate-thiabendazole-dexamethasone), which has ovicidal and adulticidal activity, or various antibiotic-antifungal-corticosteroid otic ointments which smother the mites, such as OtomaxÂ®, are effective if used for 14-21 days in combination with three to four weekly whole body applications of flea sprays to kill ectopic mites. A newer otic parasiticidal product, AcarexxÂ®, which contains 0.01 percent ivermectin, has been found to be effective against O. cynotis adults, eggs and larvae. In multiple pet households, or when otic products are ineffective or difficult to apply, systemic ivermectin (1 percent Ivomec bovine solution) is very effective at 0.3mg/kg subcutaneously every two weeks for two treatments or orally once a week for three to four weeks. Due to a higher rate of adverse neurologic reactions, ivermectin should not be used in kittens less than 16 weeks of age. Selamectin (RevolutionÂ®) or fipronil (Frontline Top SpotÂ®) applied once a month for two treatments is also very effective in the treatment of feline Otodectes infestations. If there is secondary bacterial or yeast infection, or a large amount of otic discharge, then a concurrent topical otic medication such as TresedermÂ® should be used after ear cleaning. Failure to identify and treat ear mites in a timely fashion may lead to permanent ear canal damage, with chronic otitis externa and otitis media. In addition, the ear canals' normal "self-cleaning" mechanism, whereby canal epithelial cells slide over each other to push out accumulated cerumen and debris, may be damaged. This failure of self-cleaning often leads to chronic ceruminolith (wax ball) formation deep within the ear canal later in life.
Photo 2: Microsporum canis macroconidia showing canoe-like shape and fungal hyphae.
Feline dermatophytosis is a fungal infection caused by the zoonotic dermatophyte Microsporum canis (Photo 2). This organism lives in the superficial keratinized layer of the skin, hair and claws. These fungi can only infect hairs in the anagen (growth) phase. After exposure to a dermatophyte, damage to the stratum corneum is needed for infection to occur through invasion of anagen hair follicles. M. canis is transmitted via direct contact with infected cats or kittens, or indirectly via contact with infected hair. Infection may also occur via contact with fungal elements on grooming tools, cat carriers, bedding or in the infected cat's environment (house dust, furnace filters, heating vents, carpets, drapery, floors and furniture). Arthospores of M. canis in the environment can remain infectious for up to 18 months! Asymptomatic carrier cats may occur, with passive carriage of arthospores on the haircoat acquired from an infected cat or contaminated environment. In naturally occurring dermatophytosis, up to a six-week incubation period is possible from the time of exposure to infection. In healthy kittens or cats, dermatophytosis generally is self-limiting, and often will resolve without treatment in three months. Young kittens are predisposed to this infection due to the presence of a still-developing cell-mediated immune system. Persian and Himalayan cats are predisposed to developing more severe, resistant generalized dermatophytosis, possibly due to a fungal-specific defect in cell-mediated immunity. Any disease, medication, or physiological state that weakens the host immune response can increase the risk of infection.
Examples include FeLV/FIV infection, iatrogenic or naturally-occurring hyperadrenocorticism, diabetes mellitus, neoplasia, and pregnancy/lactation. One of the worst cases of dermatophytosis that I ever saw was in a cat that had been treated with monthly Depo-MedrolÂ® injections for one year. About 50 percent of people exposed to symptomatic or asymptomatic carrier cats develop signs of dermatophytosis. Children and elderly people are predisposed, as are people on chemotherapy or other immunosuppressive medications (see Photo 4). Clinical signs of dermatophytosis in cats can be very variable, thus any cat with a dermatitis or hair loss should have a fungal culture performed as part of a routine work-up for skin disease.
Photo 4: Human dermatophytosis infection.
The most common clinical presentation is one or more circular patches of scaly alopecia with broken frayed hairs, most common on the face, head, ears and/or forelimbs (see Photo 1, and Photo 5). Other presentations of dermatophytosis include: pruritic miliary dermatitis, non-inflammatory pruritus and alopecia, chin acne, localized or generalized pruritic dermatitis, onychomycosis, and kerion formation (granulomatous firm ulcerated draining nodules). Kerion formation is most common in Persian cats. All cats with generalized disease or kerion formation should be screened for immunosuppressive diseases such as FeLV and FIV. Diagnosis of dermatophytosis is best made by fungal culture. Only 50 percent of M. canis isolates fluoresce with the use of a Wood's lamp, thus lack of fluorescence does not rule-out dermatophytosis, and a culture should always be obtained. A positive fungal culture isolate shows a red color change as the dermatophyte utilizes the protein in the dermatophyte test media first, with a simultaneous white cottony colony growth. The colonies should be identified microscopically to rule-out saprophytes. Cats with suspected kerion formation should be diagnosed via skin biopsy and histopathology. Asymptomatic cats in a household suspected of being dermatophyte carriers should be cultured using the McKenzie toothbrush technique. In this method, the cat is vigorously brushed with a new individually wrapped sterile toothbrush for two minutes over the entire body. The bristles are then impregnated gently into the dermatophyte test media, along with any hair or scales accumulated in the bristles.
Photo 1: Microsporum canis dermatophytosis lesion on the bridge of the nose of a cat.
Treatment of dermatophytosis requires long-term topical and systemic therapy, as well as environmental treatment in all cases. It is important to explain to clients that this treatment will be time-consuming, long-term and expensive, if a good result is to be obtained. Treatment often takes two to six months. The patient should be isolated in one room or in a large crate from children, elderly and immunocompromised people and non-infected (culture negative) animals. No new pets should be introduced to the household during this period of time. All culture positive animals must be treated. Most cats with localized lesions eventually develop generalized disease, thus both topical and systemic therapy is recommended to decrease shedding of infected spores into the environment. All infected cats should be bathed with an antifungal shampoo such as DermazoleÂ®, MalasebÂ® or KetochlorÂ® (avoid the eyes to prevent corneal ulcers which is possible with chlorhexidine-containing products) then dipped with 2 percent lime sulfur twice at five to seven day intervals before gently and atraumatically clipping the entire body, including the whiskers. All clipped hair must be disposed of in a biohazard sealed container. Clippers should be sterilized after using. Clipping the haircoat may temporarily worsen and spread lesions, but is needed to reduce environmental contamination. All infected cats should continue having weekly lime sulfur dips for the duration of their therapy. Spot treating localized lesions has limited value but lesions can be treated with anti-fungal lotions or creams containing miconazole or clotrimazole. Systemic therapy is recommended for most cases of dermatophytosis and in all cases of generalized disease. Micronized griseofulvin is the initial treatment of choice in most cases with the exception of cats with kerion formation, for which it is ineffective. Micronized griseofulvin is dosed at 50mg/kg/day given with a fatty meal. Side effects are uncommon and include gastrointestinal upset, teratogenicity (should not be used in pregnant queens), and irreversible idiosyncratic bone marrow suppression (most common in FIV positive cats, also may be more common in Persian, Himalayan, Siamese and Abyssinian cats), so should not be used in these cases. Regular monitoring with a complete blood count performed every two weeks is recommended on cats on griseofulvin. In cats that cannot tolerate griseofulvin, or in those with a kerion, itraconazole is the drug of choice.
Photo 5: Dermatophytosis on the head and ears of a Persian cat showing alopecia and grayish crusts.
Itraconazole is dosed at 5mg/kg/day with a fatty meal. Itraconazole needs to be compounded, is expensive, but is usually well-tolerated. Side-effects in sensitive animals include decreased appetite and increased liver enzymes. If these side-effects are seen, the drug is discontinued until the cat is eating well and liver enzymes are normal, then reinstituted at a lower daily dose or every other day. In my experience this drug is very helpful in Persian cats with griseofulvin-resistant generalized dermatophytosis. Recent evidence suggests that pulse-dosing itraconazole may be as effective as daily dosing. Anecdotally reported effective dosing protocols include: 28 days of daily therapy followed by one week on, one week off therapy for four to six more weeks or until cured, or two weeks of daily therapy followed by two days on, five days off until cured. Terbinafine, at 10-30mg/kg/day (use lower end of the dose range for kittens) has anecdotally been reported to be effective with no reported side effects by a few dermatologists, however, no long-term studies on its safety and efficacy in cats have been published. Recently leufeneron, a chitin-synthesis inhibitor used for monthly flea larvicidal/ovicidal control has received attention as a possible treatment for dermatophytosis. A dose of 70-100mg/kg (higher dose range for multi-cat households or infected catteries) given orally once and repeated in one month is reportedly effective. Anecdotal reports on efficacy vary from complete cure to ineffective. In some mild cases, this treatment may be helpful, however, studies need to be published to evaluate its true efficacy when used alone to treat dermatophytosis in larger numbers of cats. As this drug is very safe, I recommend it to be used as adjunctive therapy with other topical and systemic therapies at this time. A vaccine against M. canis was introduced in 1994. No long-term controlled studies on its efficacy have been published to date. Anecdotal reports of efficacy have been disappointing and it has been associated with sterile abscesses in some cases. Therefore, use of the vaccine to prevent or treat dermatophytosis is not recommended.
After one month of therapy, weekly fungal cultures should be obtained. Treatment must be continued for at least eight weeks, or until three consecutive weekly negative fungal cultures are obtained. Treatment of fungal kerions is difficult, and requires surgical removal followed by long-term (10-18 months) treatment with itraconazole.
Environmental control is extremely important in the eradication of dermatophytosis, since infected spores can remain viable in the environment for months or years. An extremely thorough household cleaning, continued until the cat is cured, is the only way to ensure the successful treatment of dermatophytosis without relapse. As long as there are infected spores in the house, cats can continue to carry the organism or be re-infected and transmit it back to people and other pets. The first step is a thorough vacuuming of all carpets and furniture. This is repeated twice daily if possible to remove infected hair. The vacuumed bags should be discarded after each use. Heating and air conditioning vents should be professionally cleaned and vacuumed if possible and furnace filters should be changed weekly. Carpets should be steam-cleaned to kill fungal spores. Draperies should be dry or steam-cleaned and not replaced until the infection is eradicated. Hard surfaces such as floors, baseboards, window sills, lamps, and counter tops as well as litter boxes, food/water bowels and heating/cooling vents must be disinfected at least once a week with a 1:10 solution of bleach (mopped, sponged, washed or sprayed on surfaces). Brushes, bedding, combs and toys should be disinfected with a 1:10 solution of bleach or discarded if this is not possible. If pets ride in the car, this too, must be vacuumed and disinfected. Surfaces should be completely dry and bowls rinsed well before allowing pets to contact them.
All this hard work and effort will pay off by decreasing the risk of pet and human re-infection with fungal spores, and helping to ensure successful therapy for dermatophytosis.
Feline dermatology can be both frustrating as well as challenging and rewarding. A logical stepwise diagnostic plan and offering flexible treatment options, is essential for a good outcome. Compounding pills into liquids or formulating transdermal preparations for hard to pill cats is often helpful in ensuring compliance, especially when multiple medications are given. Referral to a veterinary dermatologist for refractory or hard to manage cases before the disease has become chronic often leads to better results. Although much progress has been made in the last 10-15 years, more effective and easier to administer treatment options with fewer side effects are needed to further benefit our feline dermatology patients in the future.